1. Field of the Invention
The present invention relates to a channel changing system in a mobile unit communication system forming a communication network with multiple radio zones such that channel changing depends on transfer of a mobile unit in the busy condition to an adjacent radio zone.
2. Description of the Related Art
As illustrated in FIG. 1, a mobile unit communication system has a configuration wherein radio base stations BSl.about.BSn are arranged in a plurality of radio zones #1.about.#n, causing movable stations MS scattered therein to be capable of radio communication with such radio base stations BSl.about.BSn and with regular telephone subscribers through the one or more center stations 100 accommodating radio base stations BSl.about.BSn.
In such a mobile unit communication system, if a movable station moves under the busy condition to the adjacent radio zone from a certain radio zone, channel changing process is required for continuation of communication. FIG. 2 illustrates a diagram for explaining the busy channel changing system through the frequency changing. That is, if a movable station MS having existed in the radio zone #(n-1) moves, for example, under the busy condition, to an adjacent radio zone #n, the radio zone #(n-1) is requested to change the radio frequency fa assigned to the radio base station BS(n-1) to the radio frequency fb assigned for the use to the radio base station BSn in the radio zone #n after the movement.
Because the radio frequency to be used is changed to different frequencies respectively for each radio zone, this frequency changing system provides less frequency interference between respective radio zones and can therefore be said to be a system easily assuring management. Accordingly, the system described here is suitable, for example, to a large capacity and wide range system such as an automobile telephone system linking a plurality of zones.
The mobile unit communication system described above changes communication channels, changing, if the mobile unit transfers to an adjacent radio zone, at the region where the receiving electric field between the movable stations in the boundary of radio zones is lowered. Therefore, a protection circuit or control circuit is complicated and becomes expensive. Accordingly, such channel changing is controlled, for example, by a particular exchange control apparatus such as an automobile telephone exchanger. Such system is more suitable to a large capacity and wide range system than to a regional small scale mobile unit communication system to which the applicable region is restricted because an expensive exchange control apparatus is required.